Knocking on Heaven's Door (58 page)

But LHC experiments are not the only way to search for dark matter. The fact is that physics has now entered a potentially exciting era of data, not just for particle physics, but also for astronomy and cosmology. This chapter explains how experiments in the upcoming decade will search for dark matter using a three-pronged approach. It first explores why weak-scale-mass dark matter particles are favored, and after that, how the LHC might produce and identify dark matter particles if this hypothesis is right. We’ll then consider how dedicated experiments that are specifically designed to search for dark matter particles look for their arrival to Earth and try to register their feeble but potentially detectable interactions. Finally, we’ll consider the ways in which telescopes and detectors on the ground and in space look for products of dark matter particles annihilating in the sky. These three different ways of searching for dark matter are illustrated in Figure 78.

[
FIGURE 78
]
Dark matter searches take a three-pronged approach. Underground detectors look for dark matter directly hitting target nuclei. The LHC might create dark matter that leaves evidence in its experimental apparatuses. And satellites or telescopes might find evidence of dark matter annihilating and producing visible matter out in space.

TRANSPARENT MATTER

We know the density of dark matter, that it is cold (which is to say, it moves slowly relative to the speed of light), and that it interacts at most extremely weakly—certainly with no significant interaction with light. And that’s about it. Dark matter is transparent. We don’t know its mass, if it has any non-gravitational interactions, or how it was created in the early universe. We know its average density, but there could be one proton mass per cubic centimeter in our galaxy or there could be one thousand trillion times the proton mass stored in a compact object that is distributed throughout the universe every kilometer cubed. Either gives the same average dark matter density, and either could have seeded the formation of structure.

So although we know it’s out there, we don’t yet know the nature of dark matter. It could be small black holes or objects from other dimensions. Most likely, it is simply a new elementary particle that doesn’t have the usual Standard Model interactions—perhaps a stable neutral remnant of a soon-to-be-discovered physical theory that will appear at the weak mass scale. Even if that’s the case, we would want to know what the properties of the dark matter particle are—its mass and its interactions and if it is part of some such larger sector of new particles.

One reason the elementary particle interpretation is currently favored is the point alluded to above—the abundance of dark matter, the fraction of energy it carries—supports this hypothesis. The surprising fact is that a stable particle whose mass is roughly the weak energy scale that the LHC will explore (again via
E = mc
²
) has a relic density today—the fraction of energy stored in the particles in the universe—in the right ballpark to be dark matter.

The logic goes as follows. As the universe evolved, the temperature decreased. Heavier particles that were abundant when the universe was hotter are much more dispersed in the later cooler universe since the energy at low temperature is insufficient to create them. Once the temperature dropped sufficiently, heavy particles efficiently annihilated with heavy antiparticles so that both of them disappeared, but the reverse process where they were created no longer occurred at any significant rate. Therefore, due to annihilation, the number density of heavy particles decreased very rapidly as the universe cooled down.

Of course, in order to annihilate, particles and antiparticles have to first find each other.
⁶⁹
But as their number decreased and they became more diffuse, this became less likely. As a consequence, particles annihilated less efficiently later in the universe’s evolution since it takes two of them in the same place to tango.

The result is that substantially more stable, weak-mass particles could remain today than a naive application of thermodynamics would suggest—at some point both particles and antiparticles became so dilute that they just couldn’t find and eliminate each other. How many particles are left today depends on the mass and the interactions of the putative dark matter candidate. Physicists know how to calculate the relic abundance if we know these quantities. The intriguing and remarkable fact is that stable weak-mass particles happen to be such that they are left with about the right abundance to be the dark matter.

Of course, since we know neither the exact particle mass nor the precise interactions (not to mention the model of which this stable particle might be a part), we don’t yet know if the numbers work out exactly. But the fortuitous, albeit rough, agreement between numbers associated with what on the surface appear to be two entirely different phenomena is intriguing, and might well be a signal that weak-scale physics accounts for the dark matter in the universe.

This type of dark matter candidate has become generically known as a
WIMP
, or a
Weakly Interacting Massive Particle
. The word “weak” here is a descriptive term and not a reference to the weak force—a WIMP would interact even more weakly than the Standard Model’s weakly interacting neutrinos. Without more direct evidence for dark matter and its properties of the sort the LHC might reveal, we won’t know whether dark matter indeed consists of WIMPs. This is why we need experimental searches such as those we now consider.

DARK MATTER AT THE LHC

The intriguing possibility of producing dark matter is one reason cosmologists are curious about the physics of the weak energy scale and what the LHC might find. The LHC has just the right energy to search for a WIMP. If dark matter is indeed composed of a particle associated with the weak energy scale as the above calculation suggests, it just might be created at the LHC.

Even if that’s the case, however, the dark matter particle won’t necessarily be discovered. After all, dark matter doesn’t interact a lot. Due to their limited interactions with Standard Model matter, dark matter particles certainly won’t be produced directly or found in a detector. Even if produced, they will just fly through. Nonetheless, all is not lost (even if the dark matter particle will be). Any solution to the hierarchy problem will contain other particles—most of which have stronger interactions. Some of these might be copiously produced and subsequently decay into dark matter that will then carry away undetected momentum and energy.

Supersymmetric models are the most well-studied weak scale models of this type that naturally contain a viable dark matter candidate. If supersymmetry applies in the world, the lightest supersymmetric particle (LSP) might constitute the dark matter. This lightest particle, which carries zero electric charge, interacts too weakly to be produced on its own sufficiently often to find. However, gluinos—supersymmetric partners of the strong-force-communicating gluons, and squarks—supersymmetric partners of quarks—would be created if they exist and are in the right mass range. And, as was discussed in Chapter 17, both of these supersymmetric particles would eventually decay into the LSP. So even though a dark matter particle wouldn’t be produced directly, decays of other more prolifically created particles could conceivably create LSPs at an observable rate.

Other weak-scale dark matter scenarios that have testable consequences would have to be produced and “detected” in much the same way. The mass of the dark matter particle should be around the weak scale energy that the LHC will study. Those particles won’t be produced directly because of their feeble interaction strength, but many models contain other new particles that could decay into them. We might then learn of the dark matter particle’s existence and possibly its mass through the missing momentum it carries away.

Finding dark matter at the LHC would certainly be a major accomplishment. If it is found there, experimenters could even study some of its properties in detail. However, to really establish that a particle found at the LHC indeed constitutes the dark matter would require supplementary evidence. That is what detectors on the ground and in space might provide.

DIRECT DETECTION DARK MATTER EXPERIMENTS

The LHC’s potential to create dark matter is certainly intriguing. But most cosmology experiments don’t take place at accelerators. Experiments on Earth and in space that are dedicated to astronomical and dark matter searches are primarily responsible for addressing and advancing our understanding of potential solutions to cosmological questions.

Of course, dark matter’s interactions with matter are very weak, so current searches rely on a leap of faith that dark matter, despite its near invisibility, nonetheless interacts feebly—but not impossibly so—with matter we know (and can build detectors out of). This isn’t merely a wishful guess. It’s based on the same relic density calculation mentioned above that shows that if dark matter is related to models proposed to explain the hierarchy problem, then the density of particles that remains is the correct amount to account for dark matter observations. Many of the WIMP dark matter candidates suggested by this calculation interact with Standard Model particles at rates that might well be detectable with the current generation of dark matter detectors.

Even so, because of dark matter’s feeble interactions, the search requires either enormous detectors on the ground or, alternatively, very sensitive detectors that look for the products of dark matter meeting, annihilating, and creating new particles and their antiparticles on Earth or in space. You probably wouldn’t win the lottery if you bought only a single ticket, but if you could buy more than half of what’s available, then your odds would be pretty good. Similarly, very large detectors have a reasonable chance of finding dark matter, even though dark matter’s interaction with any single nucleon in the detector is extremely small.

The challenging task for dark matter detectors is to detect the neutral—uncharged—dark matter particles, and afterward distinguish them from cosmic rays or other background radiation. Particles with no charges don’t interact with detectors in conventional ways. The only trace of a dark matter particle passing through a detector would be the consequences of hitting nuclei in the detector and changing its energy by a minuscule amount. Because this is the only observable consequence, dark matter detectors have no choice but to search for evidence of the tiny amounts of heat or recoil energy that get created when dark matter particles pass through. Detectors are therefore designed to be either very cold or very sensitive in order to record the small heat or energy deposits from dark matter particles subtly ricocheting off.

The very cold devices, known as
cryogenic detectors
, detect the small amount of heat emitted when a dark matter particle enters the apparatus. A small amount of heat added to an already hot detector would be too difficult to notice, but with specially designed cold detectors, the tiny heat deposit can be absorbed and recorded. Cryogenic detectors are made with a crystalline absorber such as germanium. Experiments of this sort include the Cryogenic Dark Matter Search (CDMS), CRESST, and EDELWEISS.

The other class of direct detection experiments involves noble liquid detectors. Even though dark matter doesn’t directly interact with light, the energy added to an atom of xenon or argon when a dark matter particle hits it can lead to a flash of characteristic scintillation. Experiments with xenon include XENONIOO and LUX, and the other noble liquid experiments, ZEPLIN and ArDM.

Everyone in the theoretical and experimental communities is eager to know what the new results from these experiments will be. I was fortunate to be present at a dark matter conference at the KITP in Santa Barbara organized in December 2009, by two leading dark matter experts, Doug Finkbeiner and Neal Weiner, when CDMS, one of the most sensitive dark matter detection experiments, was about to release new results. In addition to being young and tall contemporaries who had done their PhDs together at Berkeley, Doug and Neal both had a great understanding of dark matter experiments and what their implications might be. Neal had more of a particle physics background, and Doug had done more astrophysics research, but they converged on the topic of dark matter when it became clear that dark matter studies would involve both. At the conference, they had collected leading theoretical and experimental expertise on the subject.

The most riveting talk of the day occurred the morning I arrived. Harry Nelson, who is a professor at the University of California Santa Barbara, talked about year-old CDMS results. You might wonder why a talk about old results should receive so much attention. The reason was that everyone at the conference knew that only three days later the experiment would release new data. And rumors were flying that scientists at the CDMS experiment had actually seen compelling evidence of a discovery, so everyone wanted to understand the experiment better. For years theorists had listened to talks about dark matter detection but had listened primarily to their results and had paid only superficial attention to the details. But with imminent dark matter detection conceivable, theorists were eager to learn more. Later in the week, the results were released and disappointed the audience’s greatly exaggerated expectations. But at the time of the talk, everyone was absorbed. Harry steadfastly managed to give his talk despite the many probing questions about the soon-to-be-released results.